4 Answers
4

Birds don't sing in a key. Keys are human cultural constructions with some basis in physics, but primarily just artful constructions of a sentient mind.

In fact, when you hear a parrot or something similar attempt to mimic human song, it's not nearly as "in key" as even an average human singer would be.

Birdsong only sounds pleasant because you've learned to associate the sound of birdsong with pleasant things. This is clearly not uncommon! But don't confuse "pleasant-sounding" with "in key." They are NOT the same. A babbling brook, I might say, is a pleasant sound--but it's not even something that can be easily defined in pitch.

Now, what you may be hearing are recognizable intervals between notes being sounded by what I'd call a monophonic pitched songbird. It might be possible to interpret groups of these notes as if they were in a key, and in fact some composers are known to do this (see Olivier Messiaen). However, intervals between pairs of notes don't put birdsong into a key, since every next interval is going to imply something totally different. The intervals themselves are probably based on the harmonic series (since that's the easiest way for physics to work), which is a shared root attribute of human cultural tonality, hence why those notes can sounds like they're "in key".

A lot of information compressed into small paragraph. I would like to asked thing from top-down. I do not understand by "Birdsong only sounds pleasant because you've learned to associate the sound of birdsong with pleasant things." -- what pleasant things? This line can be used with anything that we say 'we like'.
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iamcreasyJun 6 '12 at 3:04

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I agree - I think the second explanation (namely, that birdsong sounds more or less in key because that's how the physics of sound production and reception works) holds more water, and birdsong with recognizable tones probably sounds pleasant to us because we produce and recognize sound in a similar way.
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reinierpostJun 6 '12 at 5:27

Birds tend to only sing in good weather when the sun is shining. This I think is the most important aspect to "associating birdsong with pleasant things".
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aweJun 6 '12 at 6:05

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I probably could have written 'pleasant feelings' instead of 'pleasant things', that is, typical human beings have developed a positive behavioral response to the sound of birdsong. If a sparrow made the exact same sound but was a gigantic carnivorous beast that ate humans for dinner, you would have developed a negative behavioral response to the same sound. This is basic natural selection, and can be applied to why we think babies are cute, why healthy people are more attractive, and why sweets taste good.
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NReilingh♦Jun 7 '12 at 2:15

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Re: association: birds only 'sing' when there are no predators around. If there are, birds usually screech and chatter. Veldt evolution in action? Also, if you've aver been around a large mass of birds of the same species all making noise together, you'll know it's not always pleasant.
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naught101Oct 11 '12 at 8:17

It seems to me that Birds do not sound out of key or off pitch, what have you, when they sign, because they are higher up in the register where the scale length is getting tighter and it is harder to distinguish off key notes since the intervals are smaller. They have a harmonic overtone and this causes it to be more melodic in the sense that it is more harmonic even though we know melody and harmony are not the same. Reminds me of a story about the Fox and the Crow and the Fox gets the Crow in the Tree to sing for him so that he drops his cheese. Suddenly I feel I may have fallen victim to this very scheme at this very moment as I am trying to sign this tune of why birds have fancy wings.... But I Digress.

Also as pictured in the waveform and frequency response curves the sound consists of a more square or sawtooth type wave than a pure sinusoidal wave which accounts for the chirp.

Truth is the birds have no sense of melody and are horribly off key. The randomly improvise with chaotic tweeting and twittering that has no rhythmic sensibility and would not qualify as Chopin. Perhaps that is just me though... No ??
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Daniel BuchananNov 16 '13 at 4:18

Maybe I should try to test your theory that "they don't sound off-key because they sing so high" by transposing a bird song by an octave or few.
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Jan DvorakNov 16 '13 at 4:23

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If you look at the frequency plots it is quite obvious that this theory doesn't have any basis in fact. It is not more difficult to distinguish off pitch notes (as you can test for yourself by transposing any piece of music up a couple of octaves)
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Dr Mayhem♦Nov 16 '13 at 12:51

I'm happy to admit that I don't know what I'm talking about on this subject but its my understanding that all creatures that rely to some extent on sonic communication occupy very select frequencies on the global soundscape.

i.e. Elephants use very low bass frequencies to communicate, whales very high frequencies. It depends on the medium in which the animals exist i.e. air or water etc.

Birds themselves communicate on different bandwidths of frequency.

A sparrow for instance communicates sings on a bandwidth that is distinct from say, a magpie. This is so when they all occupy the same space they can differentiate the calls of their own species from that of others i.e. increase the signal to noise ratio.

No doubt their hearing is specifically attuned to these frequencies as well. I'm not saying they're deaf to other frequencies but that their hearing is geared towards a specific range of frequencies as is our own.

So when you ask how is this possible. Evolution no doubt. I'm sure their lyranx or tweeters or whatever you call a birds voice box is engineered to produce certain frequencies in much the same way as tuning a guitar... by tensioning the strings.

In other words its possible because its necessary to survive.
If birds couldn't call a mate or hear the cries of their young when in a forest full of different bird species, they'd simply perish.

I don't know when the last time was that you were in a forest but them birds, they sure do make a racket. They're always talking over the top of one another. So without "perfect pitch" they may as well be talking to themselves.